1. Field of the Invention
The present invention relates to a diffraction optical element and an optical system having the diffraction optical element.
2. Related Background Art
Conventionally, aside from a method with which chromatic aberrations of a lens system are reduced through combination of glass materials, a method is known with which such chromatic aberrations of a lens system are reduced by providing a diffraction optical element having a diffraction function for a part of a lens surface or a lens system (SPIE Vol. 1354 International Lens Design Conference (1990), Japanese Patent Application Laid-Open No. H04-213421 (corresponding to U.S. Pat. No. 5,044,706), Japanese Patent Application Laid-Open No. H06-324262 (corresponding to U.S. Pat. No. 5,790,321), and U.S. Pat. No. 5,044,706). The method using a diffraction optical element is a method utilizing a physical phenomenon where the appearing direction of chromatic aberrations with respect to a light beam at a certain reference wavelength at a refractive surface and the appearing direction thereof at a diffraction surface become opposite to each other in an optical system.
Also, it is possible to give an aspherical lens effect to the diffraction optical element by appropriately changing the period of the periodic structure thereof, so the diffraction optical element is also effective at reducing various aberrations other than chromatic aberrations.
In a lens system having a diffraction optical element, when light fluxes in a usage wavelength range are concentrated on diffraction light on a specific order (hereinafter also referred to as “specific order” or “design order”), the intensity of diffraction light on the other orders is reduced and when the intensity becomes “0,” this results in a situation where the diffraction light does not exist. In reality, however, unnecessary diffraction light on such orders other than the design order exists and when the unnecessary diffraction light has certain intensity, it travels in an optical system through different path from that of a light beam on the design order and becomes flare light.
Consequently, in order to utilize an aberration reduction function by utilizing a diffraction optical element, it is necessary that the diffraction efficiency as to diffraction light on the design order be sufficiently high in the entire usage wavelength range. It is important that consideration is sufficiently given to the spectral distribution of the diffraction efficiency on the design order and the behavior of the unnecessary diffraction light on the orders other than the design order.
Therefore, various diffraction optical elements are proposed which each have a construction where diffraction efficiency is improved and unnecessary diffraction light is reduced (Japanese Patent Application Laid-Open No. H09-127322 (corresponding to U.S. Pat. No. 6,157,488), Japanese Patent Application Laid-Open No. 2000-098118 (corresponding to U.S. Pat. No. 6,560,019), Japanese Patent No. 03495884 (corresponding to U.S. Pat. No. 6,480,332). Diffraction optical elements disclosed in Japanese Patent Application Laid-Open No. H09-127322, Japanese Patent Application Laid-Open No. 2000-098118, and Japanese Patent No. 03495884 realize high diffraction efficiency in a wide wavelength band (around 98% in a wavelength range of 450 nm to 650 nm) for diffraction light on a desired order by lamination-arranging multiple diffraction gratings and appropriately setting the material of each diffraction grating and the height of each diffraction grating (such a diffraction optical element will be hereinafter referred to as “laminated DOE”). Note that the diffraction efficiency is expressed by a ratio of the light amount of diffraction light on each order to the total amount of all transmission light beam.
In Japanese Patent No. 03495884, a construction is disclosed with which changes in diffraction efficiency resulting from changes in the refractive index of a material of a diffraction grating due to temperature changes are reduced. More specifically, through optimum settings of refractive index changes due to temperature changes of two materials forming respective diffraction gratings and the grating heights of two diffraction gratings, diffraction efficiency changes are suppressed in a range of a temperature change of 30° C. With this construction, from the viewpoint of not only improvement of diffraction efficiency in terms of design but also stabilization of performance under a usage environment, optimization of the materials forming the diffraction gratings and the shapes of the diffraction gratings is performed.
Recently, a diffraction optical element whose diffraction efficiency is further improved is proposed (Japanese Patent Application Laid-Open No. 2004-78166 (corresponding to U.S. 2003-0231396)). Japanese Patent Application Laid-Open No. 2004-78166 discloses a diffraction optical element in which a fine particle dispersed resin is adopted as a material of a diffraction grating in which inorganic fine particles are mixed into a resin material while giving consideration to the partial dispersion ratio θgF of the material. With this construction, as indicated by a curve 1 shown in FIG. 14, high diffraction efficiency of 99.8% or more is obtained in the entire usage wavelength range.
In addition, with the diffraction optical element disclosed in Japanese Patent Application Laid-Open No. 2004-78166, as indicated by a curve 1 shown in each of FIGS. 20A, 20B, 21A, and 21B, the diffraction efficiency of diffraction light on orders of “design order±1” that are unnecessary diffraction orders (zero-order diffraction light and +2nd-order diffraction light) is favorably suppressed to 0.05% or less in the entire usage wavelength range. As a result, the diffraction optical element disclosed in Japanese Patent Application Laid-Open No. 2004-78166 reduces the unnecessary diffraction light to around 1/10 in comparison with a diffraction optical element using a conventional material.
With the diffraction optical element disclosed in Japanese Patent Application Laid-Open No. 2004-78166, the diffraction efficiency is significantly improved, but it is further preferable that deviations of the diffraction efficiency due to usage environments be suppressed.
With the diffraction optical element disclosed in Japanese Patent No. 03495884, change in diffraction efficiency on the design order due to 30° C. change in temperature is suppressed to around 2%. Changes by 2% in the diffraction efficiency on the design order means that the diffraction efficiency of 99.8% which has been improved by the diffraction optical element disclosed in Japanese Patent Application Laid-Open No. 2004-78166 is degraded to around 98% due to temperature changes. The diffraction efficiency of the diffraction optical element before the improvement using the technique disclosed in Japanese Patent Application Laid-Open No. 2004-78166 is around 98% in a wavelength range of 450 nm to 650 nm, therefore, the diffraction efficiency improved through the technique disclosed in Japanese Patent Application Laid-Open No. 2004-78166 might be nullified.
Curve 2 shown in each of FIGS. 14 to 16 represents the diffraction efficiency in the case where the refractive index of the material changes due to temperature change of 30° C. As is apparent from FIGS. 14 to 16, the diffraction efficiency of diffraction light on the +1st-order that is the design order is decreased by around 0.5 to 2.0% and the diffraction efficiency of the zero-order diffraction light and the 2nd-order diffraction light that are unnecessary diffraction light are conversely increased by around 0.2 to 0.7%.
It should be noted here that the above description has been made by taking, as an example, a change in the diffraction efficiency due to temperature deviation, but it is also possible to consider a situation where the optical characteristic of a material forming a diffraction grating changes due to moisture absorption. That is, it is important that consideration is given to change in the diffraction efficiency resulting from environmental deviations including a moisture deviation and the like.
In reality, the degree of the change in the diffraction efficiency resulting from the environmental deviations changes in behavior to some extent in accordance with the initial diffraction efficiency and the combination of grating shapes. However, the change in the diffraction efficiency due to the environmental deviations fundamentally depends on the characteristic of the material, so it becomes important to improve the environmental tolerance characteristic of the material.
When it is possible to suppress the deviation of the curve 2 shown in FIG. 14 to a deviation of around 0.2%, even when environmental changes exist, it is possible to say that the diffraction efficiency is sufficiently high. The environmental tolerance characteristic of the material can be conversely obtained therefrom, for example, the refractive index change described above should be improved to ¼ or ⅛ of a general resin. This corresponds to changing the fundamental physical properties of the resin material, which is difficult to realize.
When a material other than a resin, more specifically, glass or the like is used, it is possible to significantly improve refractive index changes resulting from environmental changes to around 1/10 of a case of the resin. In this case, however, it is required to produce every material of a laminated DOE using glass and it is impossible to use a material, such as the material disclosed in Japanese Patent Application Laid-Open No. 2004-78166, where fine particles are dispersed. Therefore, a novel material for improvement of initial performance becomes necessary.